Simplifications

Hi @hinerm 

thanks for implementing it. I made some simplifications to your add-ons. The method for updating the progress bar and the drawing seemed a bit convoluted and I do not want to have synchronized blocks if not absolutely necessary. 

Done:
- Just the first thread updates progress bar and draws, less overhead, could removed all the synchronized blocks and complicated methods
- just tested if to update for both (bar & image) together every 10000 pixels
- remove the progress bar once stitching is done
- added the option of display also to the pairwise stitching
- move the static boolean to the GUI class and the boolean for the current stitching to StitchingParameters

If you have any thoughts let me know.
Thanks again,
Stephan